1. Field of the Invention
The present invention relates to a solar cell module which is highly reliable at a low cost, as well as to a production method and an installation method therefor, and a photovoltaic power generation system. In particular, the present invention relates to a roof material-integrated type solar cell module which has undergone transformation processing so as not to influence its electrical characteristics and which can be excellently designed, and a production method therefor.
2. Related Background Art
In the midst of increasing global environmental problems in recent years, solar photovoltaic energy has become keenly noted as clean energy which does not produce harmful by-products such as those produced in connection with thermal power and nuclear power, etc. In addition, given the limited resources on the earth, efficient use of solar energy which is an unlimited energy source is highly desired.
In addition, a centralized energy system such as with thermal power or nuclear power presents problems such that energy supply might be disturbed or might take an enormous time for recovery when a disaster such as an earthquake takes place. Solar energy can be utilized wherever and whenever it is sunny, and therefore has a high utility value as a dispersion type independent energy source.
These needs have promoted a development of a solar cell module which can be used on house roofs, and today regulatory systems for installation of photovoltaic power generation systems and operation thereof have been prepared.
The photovoltaic power generation system requires, as a power source thereof, a solar cell array composed of a plurality of connected solar cell modules. As the system for a general houses, a photovoltaic power generation system of 3 kw is standard, and in this case, almost all portions of the roof plane facing south are occupied with solar cell arrays.
The structures of disposing solar cell arrays on the roof of a house are roughly divided into two kinds, namely, the frame-installation type structure and the roof material-integrated type structure. Each of them is described as follows.
The frame-installation type structure comprises a frame and solar cell panels, wherein the frame is disposed on the roof and the solar panels are arranged thereon. Therefore, there is an advantage that solar cell arrays can be disposed on an existing roof. However, the weight of the entire roof will be heavy, and earthquake proofness is reduced; in addition, the cost for installing the frames and the solar cell panels increases.
The roof material-integrated type structure is composed of a roof material-integrated type solar cell module. The roof material-integrated type solar cell module is superior in compatibility with normal roofs since solar cells are integrated with a conventional roof material to form a module. For example, also as for installation, the same construction method as that for conventional roof material can be employed, and in addition, furnishings such as clips, etc. which are necessary to fix roof materials can be used. In addition, since the roof material-integrated type structure does not require frames and since solar cell arrays can be obtained only by roofing, the costs for installation are low. Furthermore, the roof weight is far lighter and more excellent in earthquake proofness than that in the frame-installation type structure. In addition, integration with the roof materials is harmonized with the roof design, which is an excellent advantage from the aesthetic point of-view.
As described above, the roof material-integrated type solar cell module provides many advantages, and the present inventors are proceeding with research and development so as to attain its practical use.
In Japanese Patent Application Laid-Open No. 7-302924, a roof material-integrated type solar cell module is described. For the roof material-integrated type solar cell module, in terms of its production method, a plane-shaped solar cell module can be processed with a roller former molding apparatus for conventional roof materials, and thus no further equipment investments are required and its production can be implemented at a low cost. This solar cell module described in Japanese Patent Application Laid-Open No. 7-302924 is formed by insulation-sealing a flexible amorphous silicon semiconductor with a resin on a steel plate conventionally used as the roof material, in order to have a structure which can be processed in the same manner as the processing of roof materials. The roof material is designed so that roof materials for horizontal roofing is employed, and the photovoltaic elements are disposed in a flat portion. It therefore has a structure that the photovoltaic elements is subjected to strain.
The roof material-integrated type solar cell module described above will be described with reference to attached drawings. FIGS. 10A and 10B are a perspective view and a sectional view of a representative roof material-integrated type solar cell module, respectively. FIGS. 10A and 10B show a front surface member 1001, a sealing material 1002, a photovoltaic element or a photovoltaic element group 1003, a back surface insulating material 1004 and a back surface member 1005.
More specifically, the front surface member 1001 is, for example, ETFE (ethylene-tetrafluoroethylene copolymer) film, and the sealing material 1002 is, for example, EVA (ethylene-vinyl acetate copolymer). The sealing material 1002 at the light-receiving surface side is impregnated with a surface protection reinforcement (not shown in the drawings) to prevent external scratches. As the surface protection reinforcement, in particular, for example, glass nonwoven fabric is used. The photovoltaic element 1003 is, for example, an amorphous silicon semiconductor element, and in addition, the back surface insulating material 1004 is, for example, a PET (polyester) film. As the back surface member 1005, for example, a zinc-coated steel plate is used.
Each of the structural materials is a material having a property to be easily processed and therefore is processed to form a suitable shape as the roof material by plastic deformation of a zinc-coated steel plate as the back surface member. Photovoltaic element portions also can be processed and are made to remain flat taking the design of roof materials for horizontal roofing into consideration.
However, recently, individual originality tends to be deemed important, and this trend is not an exception for building materials and solar cell modules. For the purpose of producing solar cells or building materials in various shapes, as described in Japanese Patent Application Laid-Open No. 7-302924, it is necessary for such a solar cell module to be processed over all regions including its photovoltaic elements, thereby making it possible to design without always keeping surfaces of photovoltaic elements flat.
As an example corresponding to this, a solar cell module in a wave shape is described in Japanese Patent Publication No. 6-5769. The main purpose of adopting a wave shape is to increase efficiency for light utilization. The production method includes procedures of first producing a solar cell module having flexibility and next bonding it onto a wave-shaped steel plate with an adhesive.
However, in the wave-shaped solar cell module described in Japanese Patent Publication No. 6-5769, photovoltaic elements are arranged in wave shapes, but no consideration has been paid to concrete stress to be put onto the photovoltaic elements as well as the influence on the electrical characteristics thereof, and the reliability thereof. Furthermore, according to the production method of the above wave-shaped solar cell module, a conventional roof-material molding machine can not be used for processing and the cost reduction expected for roof material-integrated type solar cell modules has not been achieved.
On the other hand, characteristics in the case where an amorphous silicon semiconductor is deformed have been reported. For example, in Appl. Phys. Lett. 54(17), 1989, p. 1678-1680, "Electrical Properties of Hydrogenated Amorphous Silicon Layer on Polymer Film Substrate under Tensile Stress", changes in resistance of amorphous silicon layer in a dark state in the case where a single layer of amorphous silicon (0.5 .mu.m thick, and mainly composed of i-type amorphous silicon) is stacked on a PET substrate (100 .mu.m thick), and then the amorphous silicon layer is tensed. Details of this report are as follows: "When an amorphous silicon layer is pulled, its resistance raises gradually up to 7000 u.epsilon. due to the piezo effect (in a reversible fashion), and from 7000 p.epsilon., the resistance raises suddenly due to cutting of weak Si--Si bonding (in irreversible fashion). However, the amorphous silicon layer in which resistance has been raised due to strain not less than 7000 u.epsilon. returns to the original shape by annealing in 150.degree. C. for one hour."
In addition, in J. Appl. Phys. 66 (1), 1989, p. 308-311, "Effect of Mechanical Strain on Electrical Characteristics of Hydrogenated Amorphous Silicon Junction", the piezo effect of amorphous silicon having a pin junction has been reported. Contents of this report are as follows: "In amorphous silicon having pin junction, when it is subjected to strain in a parallel direction to the pin junction, electric current is decreased by eight percent in both a forward direction and a backward direction under compression stress of 7500 .mu..epsilon. (in a dark state). In addition, electric current is increased by eight percent under compression stress of 7500 .mu..epsilon.."
However, in any of those reports, no descriptions on a case where strained photovoltaic elements are used, nor descriptions on a processing method thereof and moreover reliability thereof have been made.
In addition, Japanese Patent Application Laid-Open No. 9-177274 discloses in particular a method of bending a flexible photovoltaic element. By providing a bending-controlling member on collector electrodes so that the collector electrodes do not bend in a parallel direction to the longitudinal direction of the collector electrodes, the phenomenon that highly stiff collector electrodes become unable to follow a bent surface and are peeled off from the transparent electrode layer is prevented.
However, the above application relates only to preventing highly stiff collector electrodes from being peeled off from the transparent electrode layer, but there is no description with regard to a state of a semiconductor photoactive layer or a transparent electrode layer at the time when the electrodes have been bent. In addition, in an actual case of processing of photovoltaic elements, the element may not be bent but stretched flatly, and reliability in deformation of photovoltaic elements and influence of the semiconductor photoactive layer or the transparent electrode layers due to bending have not been considered.
In addition, Japanese Patent Application Laid-Open No. 4-266069 discloses a concrete method of bending a flexible photovoltaic element. A reliable solar cell module without giving rise to deterioration in electrical characteristics is obtained by processing a photovoltaic element having a predetermined concaveconvex surface with elongating deformation of not more than six percent.
As described above, there are conventional examples of a solar cell module in which photovoltaic elements are subjected to strain, but descriptions with regard to a method of processing or with regard to reliability on photovoltaic elements subjected to strain are scarce. As a result of this, it is a fact that a low-cost roof material-integrated type solar cell module which can be obtained by simply processing a flat plate solar cell module with a molding machine has not yet been successfully put into practical use.
Under the circumstances, the present inventors have developed a solar cell module in which the photovoltaic element portion is processed. The goal therein is to obtain a solar cell module which can be processed and deformed using a conventional roof-material molding machine in any region regardless of the location of the photovoltaic elements, that is, in a free region, thereby providing a highly-designed roof material-integrated type solar cell module at a low cost.
To realize the above-described goal, it is indispensable to secure reliability in the solar cell module in the case where a flat plate solar cell module is processed, but the reliability largely depends on the strain generated at the time when the solar cell module is processed.
Among strains, a significant strain in the present invention refers to the strain related to the photovoltaic element inside the solar cell module. The strain in the photovoltaic element influences the electrical characteristics as well as reliability of the solar cell module, for example, if it is flat in outward appearance without any problems on its surface.
In Japanese Patent Application Laid-Open No. 4-266069, the relationship between a concave-convex shape and electrical characteristics of photovoltaic elements due to strain is described, but moreover in the present invention, the goal has been set to clarify the relationship between the direction of the convex-concave shape and the direction of strain, and to secure higher reliability in the processed photovoltaic elements.
FIGS. 11A and 11B are respectively a plan view and a sectional view of a representative photovoltaic element.
This photovoltaic element 1106 is composed of a semiconductor photoactive layer 1102 and a transparent conductive layer 1103 formed on a flexible substrate 1101 in this order, and further collector electrodes 1104 and a busbar electrode 1105 formed on the transparent electrode layer 1103.
When the photovoltaic element 1106 receives light, electricity is generated from the semiconductor photoactive layer 1102 with the flexible substrate 1101 and the transparent electrode layer 1103 being the poles. In that case, the flexible substrate 1101 is a conductive substrate. At the side of the transparent electrode layer 1103, electricity is collected by the collector electrode 1104, and then concentratedly flows into the busbar electrode 1105 for taking out electricity outside. The side of transparent electrode layer 1103 is formed so as to reduce the shadowed area as much as possible by miniaturizing the collector electrodes 1104 and the busbar electrode 1105 and the like for the purpose of introducing much light into the semiconductor photoactive layer 1102.
Reliability in the case where strain is added to a representative photovoltaic element as mentioned above will be described.
In this case, the photovoltaic element is deformed to have a curved surface, or it remains flat. But in any case, each constituent member is stretched and deformed by strain applied in the stretching direction. When strain is small with a value not more than the critical value at which cracking is caused for each constituent member, each constituent member is stretched and can follow deformation. However, when strain becomes larger, each constituent member is deformed in excess of the critical value at which cracking is caused, thereby generating cracks.
The flexible substrate having flexibility can follow deformation even when strain is applied to a certain extent, but the semiconductor photoactive layer as well as the transparent electrode layer have a comparatively small critical value at which cracking is caused, and therefore when strain larger than the critical value is applied, cracking occurs. The cracks generated in the transparent electrode layer are not significantly influential, but in the case where cracking takes place in the semiconductor photoactive layer, a conductive material intrudes into the crack thereby generating a short circuit between the flexible substrate of the photovoltaic element and the transparent electrode layer and deteriorating the electrical characteristics of the photovoltaic elements. For example, as the electricity collecting electrode, in general, highly conductive materials are used, and thus when cracks are generated under the collector electrode, a short circuit is highly possible.
Next, F. F. (fill factor) which is a factor representing electrical characteristics of the photovoltaic elements will be described.
F. F. is represented by the equation: F. F.=maximum power Pm/(short-circuit current Isc.times.opening voltage Voc). That is, as a meaning in terms of physics, the F. F. is a value showing a ratio of the maximum power Pm which can be actually taken out to the product of Voc as a value in the case of taking out only a voltage to the maximum extent and Isc as a value in the case of taking out only a current to the maximum extent. The actual value of F. F. is determined by the characteristics of the p-n junction in the forward direction. Thus, when any leakage current flows through defects included in the photoactive layer to be used or a defect generated at the time of p-n junction production or in the succeeding manufacturing steps, the F. F. decreases to reduce the output to be generated originally. That is, when cracks generated in the semiconductor photoactive layer increase, the F. F. decreases.
Thus, reliability of the photovoltaic element subjected to strain greatly depends on the cracks generated in the semiconductor photoactive layer.